Ozone, the strongest of the common disinfecting agents, was used as early as 1893 for disinfection of drinking water in Holland. By 1916, there were 49 full-scale water treatment plants in Europe using ozone. Today, more than 2,000 water treatment plants throughout the world use ozonation for disinfection. Most of these plants are in Europe, with a particularly heavy concentration in France. Today, the principal uses of ozone are found in disinfecting water or in treating sewage.
In the United States, disinfection practice has principally relied upon the use of chlorine. However, because of concerns about byproduct formation during the disinfection process using chlorine, the alternative use of ozonation has generated considerable interest because of its ability to avoid the formation of halogenated organics. Still in an early developmental stage, the understanding of the actual mechanism of ozone reaction with biological material is not yet complete.
Ozone is characterized by strong oxidizing properties. It is an unstable gas at ambient temperatures and pressures and decomposes rapidly to oxygen at temperatures above 35.degree. C. For this reason, it cannot be manufactured and packaged at a central manufacturing plant, but must be generated on site.
Ozone has a characteristic odor, which can be detected by humans at low concentrations (0.02 ppm by volume), far below the levels of acute toxicity. It is moderately soluble in water, with solubility dependent upon the temperature of the water and the concentration of the ozone in the feed gas. It is noted that although ozone has limited solubility in water, its solubility is about 10 fold greater than the solubility of oxygen in water (mole by mole comparison). Typically, low feed gas concentrations (about 1 to 8 percent by weight) are used in disinfection processes for treating water to assure virtual organism elimination (99.9% or greater) for most organisms.
As indicated above, ozone must be generated on site, and the gas stream, which generally contains about 1 to 8 percent ozone by weight, is applied to the water flow using appropriate gas/liquid contact conditions. Although ozone is highly reactive and will dissipate within a relatively brief time, ozone residual generally can be maintained for the period of time usually required for disinfection. In general, in water treatment disinfection processes, dissolved ozone residuals of approximately 0.4 to 0.5 mg/l have been typical objectives for the disinfection process lasting about 4 to 6 minutes. Using this ozonation process at a concentration of ozone of 0.4 ml/l achieves a 99.9% inactivation of Poliomyelitis viruses in 4 to 6 minutes.
In the United States, for water treatment purposes, ozone contact times ranging from about 4 to about 12 minutes are being considered. Criteria for future ozonation parameters in reactors/sterilizers will be affected by EPA CT criteria that will require consideration of hydraulic distribution characteristics within the individual reactor or configuration. These criteria are summarized in Appendix O of the U.S. EPA Guidance Manual for Compliance with the Filtration and Disinfection Requirements for Public Water Systems Using Surface Water Sources.
Ozone is thought to achieve disinfection largely through oxidation reactions that damage and destroy critical components of microorganisms. Ozone has been recognized as an effective disinfectant for a wide range of pathogens and is applicable for achieving the primary disinfection goals for the pathogen categories regulated in the EPA Surface Water Treatment Rule.